Various transducers are characterized by a structure specialized for the purpose of transforming acoustical energy into electrical energy. Such transducers are generally referred to as microphones. A designer of electronic audio apparatus may select from various types of microphones to optimize performance on the basis of one or more comparative performance characteristics, such as bandwidth, sensitivity, and linearity. Over the last decade or so, microphones of the so-called capacitance type, including a charged electret membrane, have become the transducer of choice, because of comparatively acceptable performance characteristics and low cost. In an electronic audio apparatus such as a telephone set, there is included a transmitter circuit, whereby signals representative of a user's voice may be transmitted from the telephone set by way of an associated telephone line. In the telephony industry the use of electret microphones in the transmitter circuits of telephone handsets and telephone hands free units, is now almost universal. In the transmitter circuit the electret microphone provides conversion of the user's voice to an electrical signal which is further amplified or processed in preparation for transmittal.
One of the most important aspects in the operation of any telephony system is strict adherence to standards of performance. For example, the standard for gain within the telephone transmission circuit is often specified to a predetermined level about which no more than a decibel or so of variation is preferred. In the manufacture of low cost electret microphones, one of the difficult parameters of performance to control is that of the sensitivity of the microphone to acoustical energy. Furthermore in the manufacture of telephony apparatus more than a decibel of unintended signal variation may be introduced in the transmitter circuit, by any of heat, shock or vibrational stresses, which typically occur during assembly operations. Hence one of several performance parameters, for which each telephone set is tested before it is delivered to a customer, is that of conformance to a transmission gain standard, as specified for the telephone system in which the telephone is intended to operate. In order to optimize the successful manufacture of electret microphone telephone sets, prior to assembly, electret microphones are tested to select only those which are within a narrow predetermined range of sensitivity, for subsequent assembly in the apparatus. Those electret microphones falling outside of the range are unsuitable and may be returned to the supplier or simply discarded. Hence it can be seen that if the stresses of the assembly process are somewhat variable, the acceptable range of sensitivity must be so narrow that the cost of acceptable electret microphones will rise. On the other hand, various changes directed toward reducing the variances of stresses in the already optimized assembly process might be experimented with. However such experiment comes with attendant risk and cost.
A manufacturer of telephony equipment intending to supply international markets is faced with at least several significantly dissimilar transmission performance standards. These variations in requirements may be met by component value modifications, which of course add to the overall cost of manufacture. In some instances the international manufacturer may find modifications intended for conformance to a foreign national standard to be of a cost which is prohibitive, and so will not compete with a foreign national manufacturer.
It is an object of the invention to enable the manufacturer of electronic apparatus, such as recorders, telephones and the like, to conveniently trim a sensitivity characteristic of an electret transducer, either post- assembly of the transducer, or post-assembly of the electronic apparatus which contains the transducer, whereby a uniform performance standard may be adhered to.